A switched reluctance machine (SRM) is well known in literature and its principle, theory of operation, and construction are all described in R. Krishnan, “Switched Reluctance Motor Drives”, CRC Press, 2001. An SRM have windings on its stator poles and there are no windings or magnets on the rotor poles or rotor slots. The stator windings of the SRM are wound around the individual stator teeth or poles, and they are concentric around the poles. Such concentric windings lend themselves to being wound on formers and then being inserted onto the stator poles in the manufacturing process of the SRM stator. When the number of poles become smaller, say four in the case of a two phase machine, the slot volume available for stator windings is not fully utilized.
FIG. 1 illustrates a partial view of an SRM stator with stator poles, windings on the stator poles, and volumes of space between the windings of adjacent poles. Partial stator 1 has three stator poles 2, 3 and 4; a fourth pole is not illustrated in FIG. 1. A stator winding 5 is wound around stator pole 2, a stator winding 6 is wound around stator pole 3, and a stator winding 7 is wound around stator pole 4. The rotor of the SRM is not shown in FIG. 1.
Consider a volume of space S1 between stator windings 5 and 6 of stator poles 2 and 3. Volume space S1 is unutilized and not filled with windings, so as to avoid mechanical and electrical interferences between windings 5 and 6. Similar reasoning applies to the unutilized volume space S2 between windings 6 and 7. Likewise, there are two other volume spaces within stator 1, but not shown in the FIG. 1.
A turn of winding is defined as one turn of winding around a pole and, therefore, will have two sides on each side of the stator pole. Multiple turns per pole constitute a coil or part of the phase winding. Depending on the number of poles and phases, the multiple turns may be interconnected. The SRM shown in FIG. 1 is assumed to have four stator poles and two rotor poles and has two phase windings. The windings on poles 2 and 4 are connected in series so that, together, they constitute one phase winding, Phase A. Similarly, the windings on pole 3 and its diametrically opposite stator pole (not shown in FIG. 1) are connected to form a Phase B winding. Windings 5, 6 and 7 are usually wound on the former and inserted on machine poles 2, 3 and 4, respectively, as is done for all poles and windings of the SRM. Machine-based automated winding of SRM windings may be resorted to.
The winding volume and area are constrained for a number of reasons. The windings in FIG. 1, say 5 and 6, for example, have to be identical for ease of manufacture. Therefore, their dimensions are identical and if one is bigger than the other in linear dimensions, the area and volume of the windings change, with the result that the windings will interfere with each other mechanically during insertion, given the fixed volume space between the two adjacent poles. Further the larger size can complicate an electrical insulation problem if the windings happen to overlap each other, since they will be at different voltages during their operational use, resulting in failure and short circuiting of the windings. The space that is unutilized, after accounting for the mechanical clearance between poles 2 and 3, is shown as S1 in FIG. 1. Space S2 between poles 3 and 4 is also unutilized. The discussion herein will focus on space S1, and similar and identical reasoning can be applied to all other spaces, including space S2.
Another constraint for the volume of stator windings arises from the shape of the stator. The stator may be shaped like a circle, an octagonal, or in between a square and a circle, making the area in between the stator poles not a regular surface, such as a rectangle. Such surface areas are hard to deal with for placing the windings, because of the crevice spaces and areas that have to be left out of the winding area.
The above-discussed constraints are the most severe within the conventional packing and arrangement of windings in an SRM. The conventional method of packing the windings is illustrated in FIG. 1, with a large unutilized area for winding as shown by areas S1 and S2.
The problem of manual insertion of the windings creates interference between adjacent windings, for example, between windings 5 and 6 in volume space S1. Waste of space volume in between the windings leads to many undesirable effects, such as: (1) a lower number of turns packed per pole winding, resulting in lower magneto motive force (mmf) for an operating current that leads to lower flux density and flux in the stator poles and in the output torque of the SRM, (2) a higher resistance per phase for a desired number of turns resulting from making the cross sectional area of the winding conductor accommodate more turns within the winding space volume, which results in higher resistive losses and lower efficiency in the operation of a machine, and (3) the power output of the SRM is lower than optimal both liar steady state and peak power operation, because of the lower number of turns per phase and higher resistance per phase.